Why do we need an isolated system for the law of conservation of mass to be true?

Explanation:

well an isolated system is one which is impervious to the floor of both matter and energy so to prove the conservation of mass we have to find a way to keep a constant mass in the confined space

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.